The present invention relates generally to the field of semiconductor device fabrication and in particular the invention provides an improved device structure and method of forming metal contacts in thin film semiconductor devices.
A major advantage of thin-film photovoltaic (PV) modules over conventional wafer-based modules is that series interconnection of the individual cells can be accomplished using a deposited metal film. It is known that particular metals make better contact with regions of one dopant type and in particular, aluminium which is commonly used as a metallisation layer makes good contact with n-type material, but can be unreliable when contacting to p-type material unless high temperatures are used.
However, cost is an important factor in thin film device manufacture and additional steps can add significantly to the cost particularly when they include an alignment step. Therefore, processes that avoid alignment steps, or are self aligning, provide significant advantages in low cost device fabrication.
According to a first aspect, the present invention provides a method of forming p-type and n-type contacts on a thin film semiconductor junction device having an underlying region of a first semiconductor type and an overlying region of a second semiconductor type including the steps of:
a) forming at least one dielectric layer over a free surface of the thin film device;
b) opening a first set of holes in the dielectric layer to expose the upper semiconductor region of the second semiconductor type in the locations where contacts are to be made to the second semiconductor type;
c) forming a first thin metal layer over the at least one dielectric layer and extending into the first set of openings to contact with the second semiconductor type, the first metal layer being formed of a metal selected to make reliable contact with the second semiconductor type;
d) opening a second set of holes through the first metal layer and the dielectric layer to expose a surface of the semiconductor;
e) doping the surface or surfaces of the semiconductor device exposed by the second set of holes in the dielectric layer with a dopant of the same polarity as the underlying region, the further doping extending to the underlying region and isolating the second set of holes from the upper semiconductor region;
f) forming a second thin metal layer over the first metal layer, the second metal layer extending into the openings in the at least one dielectric layer and contacting the surface or surfaces of the semiconductor material exposed by the second set of holes to thereby provide a connection to the underlying semiconductor region; and
g) for each cell in the device, forming an isolation groove through both metal layers to electrically isolate the contacts in the first set of openings from the contacts in the second set of openings.
The doping step, to dope the second set of openings may be either as a direct consequence of opening the second set of holes or as a subsequent process step. However, in the preferred embodiment, the doping of the surfaces of the second set of openings is performed as part of the opening step. Preferably, the opening created by the second opening step extends through the first metal layer, the one or more dielectric layers and through the semiconductor film to expose a supporting surface on which the semiconductor film is formed.
According to a second aspect, the present invention provides a method of forming p-type and n-type contacts on a thin film semiconductor junction device having an underlying region of a first semiconductor type and an overlying region of a second semiconductor type including the steps of:
a) forming at least one dielectric layer over a free surface of the thin film device;
b) opening a first set of holes through the at least one dielectric layer to expose a surface of the semiconductor in the locations where contacts are to be made to the underlying region of the first semiconductor type;
c) doping the surface or surfaces of the semiconductor device exposed by the first set of holes in the at least one dielectric layer with a dopant of the same polarity as the underlying region, the further doping extending to the underlying region and isolating the first set of holes from the upper semiconductor region;
d) forming a first thin metal layer over the at least one dielectric layer and extending into the first set of openings to contact with the first semiconductor type, the first metal layer being formed of a metal selected to make reliable contact with the first semiconductor type;
e) opening a second set of holes through the first metal layer and the at least one dielectric layer to expose the upper semiconductor region of the second semiconductor type in the locations where contacts are to be made to the second semiconductor type;
f) forming a second thin metal layer over the first metal layer, the second metal layer extending into the openings in the at least one dielectric layer and contacting the surface or surfaces of the semiconductor material exposed by the second set of holes to thereby provide a connection to the upper semiconductor region; and
g) for each cell in the device, forming an isolation groove through both metal layers to electrically isolate the contacts in the first set of openings from the contacts in the second set of openings.
The doping step, to dope the first set of openings may be either as a direct consequence of opening the first set of holes or as a subsequent process step. However, in the preferred embodiment, the doping of the surfaces of the first set of openings is performed as part of the opening step. Preferably, the opening created by the first opening step extends through the one or more dielectric layers and through the semiconductor film to expose a supporting surface on which the semiconductor film is formed.
According to a third aspect, the present invention provides a thin film semiconductor device comprising a thin semiconductor film formed on a transparent insulating substrate, the semiconductor film having at least an upper doped region of a first dopant type located adjacent an upper surface of the semiconductor film remote from the substrate, and an underlying doped region of a dopant type of opposite polarity to the first dopant type between the upper doped region and the substrate, at least one dielectric layer extending over the semiconductor film, a first thin layer of a first metal extending over the dielectric layer and a second thin layer of a second metal different to the first metal extending over and in contact with the first metal layer, a first set of openings being provided in the at least one dielectric layer such that the first metal layer contacts the semiconductor region of one dopant type to make electrical connection therewith in the first set of openings, and a second set of openings being provided in the at least one dielectric layer and the first metal layer such that the second metal layer extends into the second set of openings in the at least one dielectric layer and contacts a region of the semiconductor film, of the opposite dopant type, the first metal being selected to make reliable connection with the semiconductor material exposed by the first set of openings and the second metal being selected to make reliable connection with the semiconductor material exposed by the second set of openings.
In the preferred embodiment, the second set of openings extends completely through the semiconductor film to expose a supporting surface on which the semiconductor film is formed. These openings allow direct contact to the underlying semiconductor region. The preferred method of forming these openings also causes the walls of the openings to be simultaneously doped with the same dopant polarity as the underlying region to which contact is being made, thereby isolating the overlying, oppositely doped region from the surfaces of the openings. In a preferred method of forming the openings, a laser is used to melt/ablate the opening and the doped walls of the opening are formed by mixing of the material from the underlying region with the material in the walls during the formation of the openings.
The first metal layer will also form the back reflector of the photovoltaic device and is therefore preferably selected for good optical reflectivity. Alternatively the first metal layer can be made sufficiently thin that it is essentially transparent, in which case the optical reflectivity is determined primarily by the second metal layer.
In embodiments where the semiconductor is silicon, such as in thin polycrystalline silicon devices, the metals are preferably selected from aluminium, copper and nickel. As nickel and copper each make better contact with p-type silicon material than aluminium it is preferable that one of these metals be used as the first metal layer, to provide contact to a p-type region. Aluminium is adequate for connecting to n-type material and has a low melting point which assists in forming metal isolation grooves.
The at least one dielectric layer can be any one or more of the dielectric materials commonly used in semiconductor manufacture such as silicon dioxide, or silicon nitride, an organic resin such as Novolac(trademark), or a layered combination of these.
Typically, the dielectric layer is an order of magnitude thicker than the second metal layer and the first metal layer is an order of magnitude thinner than the second metal layer.
In thin silicon film photovoltaic devices, the silicon film is typically in the range of 0.5-10 xcexcm thick and is formed over a glass substrate. Preferably, the silicon film will be in the range of 1-3 xcexcm thick.
In the case of Novolac(trademark), the dielectric layer will be in the range of 1-10 xcexcm and preferably 2-5 xcexcm while the nickel or copper will be 5-20 nm thick and the aluminium will be 100-200 nm thick. A thin (100-200 nm) layer of silicon nitride is preferably formed between the Novolac(trademark) and the silicon.
Preferably, the opening of the dielectric layer to form the first and second sets of openings and the opening of the isolation grooves in the metal is performed by a laser although it is also possible to perform some of these operations by masking and etching or mechanical scribing, depending on the materials used.
It is known that some metals will make a reliable connection with a particular doped semiconductor, while other metals will make intermittent electrical connection with the same doped semiconductor or will only sometimes make connection. Still other metals will make no useful electrical connection with the given semiconductor. Further, metals which make good electrical connection with one doped semiconductor type, may not make satisfactory electrical connection to an oppositely doped semiconductor type. Throughout this specification, the term xe2x80x9creliably make electrical connectionxe2x80x9d, when used in relation to selection of metals for electrical contacts, will be taken to indicate that the metal is selected for its reliability in consistently making good electrical connection within the design parameter of the device, to the particular doped semiconductor material in use.